The PROUST hypothesis: the embodiment of olfactory cognition

The extension of cognition beyond the brain to the body and beyond the body to the environment is an area of debate in philosophy and the cognitive sciences. Yet, these debates largely overlook olfaction, a sensory modality used by most animals. Here, I use the philosopher's framework to explore the implications of embodiment for olfactory cognition. The philosopher's 4E framework comprises embodied cognition, emerging from a nervous system characterized by its interactions with its body. The necessity of action for perception adds enacted cognition. Cognition is further embedded in the sensory inputs of the individual and is extended beyond the individual to information stored in its physical and social environments. Further, embodiment must fulfill the criterion of mutual manipulability, where an agent's cognitive state is involved in continual, reciprocal influences with its environment. Cognition cannot be understood divorced from evolutionary history, however, and I propose adding evolved, as a fifth term to the 4E framework. We must, therefore, begin at the beginning, with chemosensation, a sensory modality that underlies purposive behavior, from bacteria to humans. The PROUST hypothesis (perceiving and reconstructing odor utility in space and time) describers how olfaction, this ancient scaffold and common denominator of animal cognition, fulfills the criteria of embodied cognition. Olfactory cognition, with its near universal taxonomic distribution as well as the near absence of conscious representation in humans, may offer us the best sensorimotor system for the study of embodiment.

How the evolution of air breathing shaped hippocampal function

To make maps from airborne odours requires dynamic respiratory patterns. I propose that this constraint explains the modulation of memory by nasal respiration in mammals, including murine rodents (e.g. laboratory mouse, laboratory rat) and humans. My prior theories of limbic system evolution offer a framework to understand why this occurs. The answer begins with the evolution of nasal respiration in Devonian lobe-finned fishes. This evolutionary innovation led to adaptive radiations in chemosensory systems, including the emergence of the vomeronasal system and a specialization of the main olfactory system for spatial orientation. As mammals continued to radiate into environments hostile to spatial olfaction (air, water), there was a loss of hippocampal structure and function in lineages that evolved sensory modalities adapted to these new environments. Hence the independent evolution of echolocation in bats and toothed whales was accompanied by a loss of hippocampal structure (whales) and an absence of hippocampal theta oscillations during navigation (bats). In conclusion, models of hippocampal function that are divorced from considerations of ecology and evolution fall short of explaining hippocampal diversity across mammals and even hippocampal function in humans. This article is part of the theme issue 'Systems neuroscience through the lens of evolutionary theory'.

Acrobatic squirrels learn to leap and land on tree branches without falling

Arboreal animals often leap through complex canopies to travel and avoid predators. Their success at making split-second, potentially life-threatening decisions of biomechanical capability depends on their skillful use of acrobatic maneuvers and learning from past efforts. Here, we found that free-ranging fox squirrels (Sciurus niger) leaping across unfamiliar, simulated branches decided where to launch by balancing a trade-off between gap distance and branch-bending compliance. Squirrels quickly learned to modify impulse generation upon repeated leaps from unfamiliar, compliant beams. A repertoire of agile landing maneuvers enabled targeted leaping without falling. Unanticipated adaptive landing and leaping "parkour" behavior revealed an innovative solution for particularly challenging leaps. Squirrels deciding and learning how to launch and land demonstrates the synergistic roles of biomechanics and cognition in robust gap-crossing strategies.

How Ambient Environment Influences Olfactory Orientation in Search and Rescue Dogs

Under natural conditions, an animal orienting to an air-borne odor plume must contend with the shifting influence of meteorological variables, such as air temperature, humidity, and wind speed, on the location and the detectability of the plume. Despite their importance, the natural statistics of such variables are difficult to reproduce in the laboratory and hence few studies have investigated strategies of olfactory orientation by mobile animals under different meteorological conditions. Using trained search and rescue dogs, we quantified the olfactory orientation behaviors of dogs searching for a trail (aged 1-3 h) of a hidden human subject in a natural landscape, under a range of meteorological conditions. Dogs were highly successful in locating the human target hidden 800 m from the start location (93% success). Humidity and air temperature had a significant effect on search strategy: as air conditions became cooler and more humid, dogs searched significantly closer to the experimental trail. Dogs also modified their speed and head position according to their search location distance from the experimental trail. When close to the trail, dogs searched with their head up and ran quickly but when their search took them farther from the trail, they were more likely to search with their nose to the ground, moving more slowly. This study of a mammalian species responding to localized shifts in ambient conditions lays the foundation for future studies of olfactory orientation, and the development of a highly tractable mammalian species for such research.

The navigational nose: a new hypothesis for the function of the human external pyramid

One of the outstanding questions in evolution is why Homo erectus became the first primate species to evolve the external pyramid, i.e. an external nose. The accepted hypothesis for this trait has been its role in respiration, to warm and humidify air as it is inspired. However, new studies testing the key assumptions of the conditioning hypothesis, such as the importance of turbulence to enhance heat and moisture exchange, have called this hypothesis into question. The human nose has two functions, however, respiration and olfaction. It is thus also possible that the external nose evolved in response to selection for olfaction. The genus Homo had many adaptations for long-distance locomotion, which allowed Homo erectus to greatly expand its species range, from Africa to Asia. Long-distance navigation in birds and other species is often accomplished by orientation to environmental odors. Such olfactory navigation, in turn, is enhanced by stereo olfaction, made possible by the separation of the olfactory sensors. By these principles, the human external nose could have evolved to separate olfactory inputs to enhance stereo olfaction. This could also explain why nose shape later became so variable: as humans became more sedentary in the Neolithic, a decreasing need for long-distance movements could have been replaced by selection for other olfactory functions, such as detecting disease, that would have been critical to survival in newly dense human settlements.

Caching for where and what: evidence for a mnemonic strategy in a scatter-hoarder

Scatter-hoarding animals face the task of maximizing retrieval of their scattered food caches while minimizing loss to pilferers. This demand should select for mnemonics, such as chunking, i.e. a hierarchical cognitive representation that is known to improve recall. Spatial chunking, where caches with the same type of content are related to each other in physical location and memory, would be one such mechanism. Here we tested the hypothesis that scatter-hoarding eastern fox squirrels (Sciurus niger) are organizing their caches in spatial patterns consistent with a chunking strategy. We presented 45 individual wild fox squirrels with a series of 16 nuts of four different species, either in runs of four of the same species or 16 nuts offered in a pseudorandom order. Squirrels either collected each nut from a different location or collected all nuts from a single location; we then mapped their subsequent cache distributions using GPS. The chunking hypothesis predicted that squirrels would spatially organize caches by nut species, regardless of presentation order. Our results instead demonstrated that squirrels spatially chunked their caches by nut species but only when caching food that was foraged from a single location. This first demonstration of spatial chunking in a scatter hoarder underscores the cognitive demand of scatter hoarding.

Inaccessibility of reinforcement increases persistence and signaling behavior in the fox squirrel (Sciurus niger)

Under natural conditions, wild animals encounter situations where previously rewarded actions do not lead to reinforcement. In the laboratory, a surprising omission of reinforcement induces behavioral and emotional responses described as frustration. Frustration can lead to aggressive behaviors and to the persistence of noneffective responses, but it may also lead to new behavioral responses to a problem, a potential adaptation. We assessed the responses to inaccessible reinforcement in free-ranging fox squirrels (Sciurus niger). We trained squirrels to open a box to obtain food reinforcement, a piece of walnut. After 9 training trials, squirrels were tested in 1 of 4 conditions: a control condition with the expected reward, an alternative reinforcement (a piece of dried corn), an empty box, or a locked box. We measured the presence of signals suggesting arousal (e.g., tail flags and tail twitches) and found that squirrels performed fewer of these behaviors in the control condition and increased certain behaviors (tail flags, biting box) in the locked box condition, compared to other experimental conditions. When faced with nonreinforcement, that is, frustration, squirrels increased the number of interactions with the apparatus and spent more time interacting with the apparatus. This study of frustration responses in a free-ranging animal extends the conclusions of captive studies to the field and demonstrates that fox squirrels show short-term negatively valenced responses to the inaccessibility, omission, and change of reinforcement. (PsycINFO Database Record

Olfactory Orientation and Navigation in Humans

Although predicted by theory, there is no direct evidence that an animal can define an arbitrary location in space as a coordinate location on an odor grid. Here we show that humans can do so. Using a spatial match-to-sample procedure, humans were led to a random location within a room diffused with two odors. After brief sampling and spatial disorientation, they had to return to this location. Over three conditions, participants had access to different sensory stimuli: olfactory only, visual only, and a final control condition with no olfactory, visual, or auditory stimuli. Humans located the target with higher accuracy in the olfaction-only condition than in the control condition and showed higher accuracy than chance. Thus a mechanism long proposed for the homing pigeon, the ability to define a location on a map constructed from chemical stimuli, may also be a navigational mechanism used by humans.

The evolution of self-control

Cognition presents evolutionary research with one of its greatest challenges. Cognitive evolution has been explained at the proximate level by shifts in absolute and relative brain volume and at the ultimate level by differences in social and dietary complexity. However, no study has integrated the experimental and phylogenetic approach at the scale required to rigorously test these explanations. Instead, previous research has largely relied on various measures of brain size as proxies for cognitive abilities. We experimentally evaluated these major evolutionary explanations by quantitatively comparing the cognitive performance of 567 individuals representing 36 species on two problem-solving tasks measuring self-control. Phylogenetic analysis revealed that absolute brain volume best predicted performance across species and accounted for considerably more variance than brain volume controlling for body mass. This result corroborates recent advances in evolutionary neurobiology and illustrates the cognitive consequences of cortical reorganization through increases in brain volume. Within primates, dietary breadth but not social group size was a strong predictor of species differences in self-control. Our results implicate robust evolutionary relationships between dietary breadth, absolute brain volume, and self-control. These findings provide a significant first step toward quantifying the primate cognitive phenome and explaining the process of cognitive evolution.

Fox squirrels match food assessment and cache effort to value and scarcity

Scatter hoarders must allocate time to assess items for caching, and to carry and bury each cache. Such decisions should be driven by economic variables, such as the value of the individual food items, the scarcity of these items, competition for food items and risk of pilferage by conspecifics. The fox squirrel, an obligate scatter-hoarder, assesses cacheable food items using two overt movements, head flicks and paw manipulations. These behaviors allow an examination of squirrel decision processes when storing food for winter survival. We measured wild squirrels' time allocations and frequencies of assessment and investment behaviors during periods of food scarcity (summer) and abundance (fall), giving the squirrels a series of 15 items (alternating five hazelnuts and five peanuts). Assessment and investment per cache increased when resource value was higher (hazelnuts) or resources were scarcer (summer), but decreased as scarcity declined (end of sessions). This is the first study to show that assessment behaviors change in response to factors that indicate daily and seasonal resource abundance, and that these factors may interact in complex ways to affect food storing decisions. Food-storing tree squirrels may be a useful and important model species to understand the complex economic decisions made under natural conditions.

Navigation outside of the box: what the lab can learn from the field and what the field can learn from the lab

Space is continuous. But the communities of researchers that study the cognitive map in non-humans are strangely divided, with debate over its existence found among behaviorists but not neuroscientists. To reconcile this and other debates within the field of navigation, we return to the concept of the parallel map theory, derived from data on hippocampal function in laboratory rodents. Here the cognitive map is redefined as the integrated map, which is a construction of dual mechanisms, one based on directional cues (bearing map) and the other on positional cues (sketch map). We propose that the dual navigational mechanisms of pigeons, the navigational map and the familiar area map, could be homologous to these mammalian parallel maps; this has implications for both research paradigms. Moreover, this has implications for the lab. To create a bearing map (and hence integrated map) from extended cues requires self-movement over a large enough space to sample and model these cues at a high resolution. Thus a navigator must be able to move freely to map extended cues; only then should the weighted hierarchy of available navigation mechanisms shift in favor of the integrated map. Because of the paucity of extended cues in the lab, the flexible solutions allowed by the integrated map should be rare, despite abundant neurophysiological evidence for the existence of the machinery needed to encode and map extended cues through voluntary movement. Not only do animals need to map extended cues but they must also have sufficient information processing capacity. This may require a specific ontogeny, in which the navigator's nervous system is exposed to naturally complex spatial contingencies, a circumstance that occurs rarely, if ever, in the lab. For example, free-ranging, flying animals must process more extended cues than walking animals and for this reason alone, the integrated map strategy may be found more reliably in some species. By taking concepts from ethology and the parallel map theory, we propose a path to directly integrating the three great experimental paradigms of navigation: the honeybee, the homing pigeon and the laboratory rodent, towards the goal of a robust, unified theory of animal navigation.

Sexual selection and the brain

Sex differences are intrinsically interesting, particularly in the brain. When sexually dimorphic structures mediate learning, and when such learning ability is necessary to compete for mates, then such differences are best understood within the framework of sexual selection. By categorizing recent studies of sex differences in the brain by their role in mate competition, theories of sexual selection can be used to predict and characterize the occurrence of dimorphisms among species with different mating systems.

How does cognition evolve? Phylogenetic comparative psychology

Now more than ever animal studies have the potential to test hypotheses regarding how cognition evolves. Comparative psychologists have developed new techniques to probe the cognitive mechanisms underlying animal behavior, and they have become increasingly skillful at adapting methodologies to test multiple species. Meanwhile, evolutionary biologists have generated quantitative approaches to investigate the phylogenetic distribution and function of phenotypic traits, including cognition. In particular, phylogenetic methods can quantitatively (1) test whether specific cognitive abilities are correlated with life history (e.g., lifespan), morphology (e.g., brain size), or socio-ecological variables (e.g., social system), (2) measure how strongly phylogenetic relatedness predicts the distribution of cognitive skills across species, and (3) estimate the ancestral state of a given cognitive trait using measures of cognitive performance from extant species. Phylogenetic methods can also be used to guide the selection of species comparisons that offer the strongest tests of a priori predictions of cognitive evolutionary hypotheses (i.e., phylogenetic targeting). Here, we explain how an integration of comparative psychology and evolutionary biology will answer a host of questions regarding the phylogenetic distribution and history of cognitive traits, as well as the evolutionary processes that drove their evolution.

Digit ratio predicts sense of direction in women

The relative length of the second-to-fourth digits (2D:4D) has been linked with prenatal androgen in humans. The 2D:4D is sexually dimorphic, with lower values in males than females, and appears to correlate with diverse measures of behavior. However, the relationship between digit ratio and cognition, and spatial cognition in particular, has produced mixed results. In the present study, we hypothesized that spatial tasks separating cue conditions that either favored female or male strategies would examine this structure-function correlation with greater precision. Previous work suggests that males are better in the use of directional cues than females. In the present study, participants learned a target location in a virtual landscape environment, in conditions that contained either all directional (i.e., distant or compass bearing) cues, or all positional (i.e., local, small objects) cues. After a short delay, participants navigated back to the target location from a novel starting location. Males had higher accuracy in initial search direction than females in environments with all directional cues. Lower digit ratio was correlated with higher accuracy of initial search direction in females in environments with all directional cues. Mental rotation scores did not correlate with digit ratio in either males or females. These results demonstrate for the first time that a sex difference in the use of directional cues, i.e., the sense of direction, is associated with more male-like digit ratio.

Scene complexity: influence on perception, memory, and development in the medial temporal lobe

Regions in the medial temporal lobe (MTL) and prefrontal cortex (PFC) are involved in memory formation for scenes in both children and adults. The development in children and adolescents of successful memory encoding for scenes has been associated with increased activation in PFC, but not MTL, regions. However, evidence suggests that a functional subregion of the MTL that supports scene perception, located in the parahippocampal gyrus (PHG), goes through a prolonged maturation process. Here we tested the hypothesis that maturation of scene perception supports the development of memory for complex scenes. Scenes were characterized by their levels of complexity defined by the number of unique object categories depicted in the scene. Recognition memory improved with age, in participants ages 8–24, for high-, but not low-, complexity scenes. High-complexity compared to low-complexity scenes activated a network of regions including the posterior PHG. The difference in activations for high- versus low-complexity scenes increased with age in the right posterior PHG. Finally, activations in right posterior PHG were associated with age-related increases in successful memory formation for high-, but not low-, complexity scenes. These results suggest that functional maturation of the right posterior PHG plays a critical role in the development of enduring long-term recollection for high-complexity scenes.

Effects of cue types on sex differences in human spatial memory

We examined the effects of cue types on human spatial memory in 3D virtual environments adapted from classical animal and human tasks. Two classes of cues of different functions were investigated: those that provide directional information, and those that provide positional information. Adding a directional cue (geographical slant) to the spatial delayed-match-to-sample task improved performance in males but not in females. When the slant directional cue was removed in a hidden-target location task, male performance was impaired but female performance was unaffected. The removal of positional cues, on the other hand, impaired female performance but not male performance. These results are consistent with results from laboratory rodents and thus support the hypothesis that sex differences in spatial memory arise from the dissociation between a preferential reliance on directional cues in males and on positional cues in females.

Sex differences in directional cue use in a virtual landscape

How males and females differ in their use of cues for spatial navigation is an important question. Although women and men appear to respond differently to close and distant objects, object features and the geometry of spaces, the common denominator of these sex-specific cue preferences is unknown. By constructing virtual landscapes from either directional (graded, gradient) or positional (pinpoint) cues, the authors tested the hypothesis that sex differences arise from preferences for cues that provide primarily direction or position, as predicted by the parallel map model of the cognitive map. Women and men learned a target location in the presence of either one or the other class of cues. Men were more accurate in estimating the target location overall, but the navigation accuracy difference between men and women was greater in the presence of directional cues. Our findings provide support for the parallel map model and suggest that the previously reported male advantage in the presence of distant objects and geometric cues derives from their function as directional cues.

Flexibility of cue use in the fox squirrel (Sciurus niger)

Recent work on captive flying squirrels has demonstrated a novel degree of flexibility in the use of different orientation cues. In the present study, we examine to what extent this flexibility is present in a free-ranging population of another tree squirrel species, the fox squirrel. We trained squirrels to a rewarded location within a square array of four feeders and then tested them on transformations of the array that either pitted two cue types against one cue type, the majority tests, or all cue types against each other, the forced-hierarchy test. In Experiment 1, squirrels reoriented to the two-cue-type location in all majority tests and to the location indicated by the visual features of the feeders in the forced-hierarchy test. This preference for visual features runs contrary to previous studies that report the use of spatial cues over visual features in food-storing species. In Experiments 2-5 we tested squirrels with different trial orders (Experiments 2 and 3), a different apparatus (Experiment 4) and at different times of the year (Experiment 5) to determine why these squirrels had chosen to orient using visual features in the first experiment. Like captive flying squirrels, free-ranging fox squirrels showed a large degree of flexibility in their use of cues. Furthermore, their cue use appeared to be sensitive both to changes in the test apparatus and the season in which we tested. Altogether our results suggest that the study of free-ranging animals over a variety of conditions is necessary for understanding spatial cognition.

Flexible use of spatial cues in the southern flying squirrel (Glaucomys volans)

Insects, birds, and mammals have been shown capable of encoding spatial information in memory using multiple strategies or frames of reference simultaneously. These strategies include orientation to a goal-specific cue or beacon, to the position of the goal in an array of local landmarks, or to its position in the array of distant landmarks, also known as the global frame of reference. From previous experiments, it appears that birds and mammals that scatter hoard rely primarily on a global frame of reference, but this generalization depends on evidence from only a few species. Here we examined spatial memory in a previously unstudied scatter hoarder, the southern flying squirrel. We dissociated the relative weighting of three potential spatial strategies (beacon, global, or relative array strategy) with three probe tests: transposition of beacon and the rotation or the expansion of the array. The squirrels' choices were consistent with a spatial averaging strategy, where they chose the location dictated by at least two of the three strategies, rather than using a single preferred frame of reference. This adaptive and flexible heuristic has not been previously described in animal orientation studies, yet it may be a common solution to the universal problem of encoding and recalling spatial locations in an ephemeral physical landscape.

From movement to transitivity: the role of hippocampal parallel maps in configural learning

Whether spatial learning is a special case of configural or relational learning, or whether abstract principles evolved from the concrete need to navigate in space, is a question of long-standing debate. The parallel map theory of hippocampal function offers a resolution of the debate by redefining 'spatial learning' as two parallel, geometric processes, Euclidean metric and topological. Moreover, these processes are subserved by independent hippocampal subfields that underlie two ways of representing space, the bearing and the sketch map. It is possible that configural and relational learning, like spatial learning, should also be distinguished in this way. Transitive inference, requiring the construction of a value gradient, could be analyzed as a Euclidean metric problem. In contrast, transverse patterning could be seen as a topological analysis of the relationships among discrete objects. If this interpretation is correct, lesions to the primary bearing map structure (dentate gyrus) should impair transitivity while lesions to the primary sketch map structure (CA1) should impair transverse patterning and similar topological tasks. Recent results from diverse species and tasks lend support to these predictions, suggesting that the hippocampus not only creates parallel maps but uses these maps to solve more abstract configural or relational problems.

The sectored foraging field: a novel design to quantify spatial strategies, learning, memory, and emotion

Although Norway rats are naturally gregarious, males typically live alone at some point during adulthood. Different social ecologies often require different learning strategies and also modulate response to stressors and gonadal development. To measure effects of the social environment on the interaction between cognition and emotion during aging, we focused on a natural learning context and devised the sectored foraging field, a progressively difficult spatial navigation task. Here, we describe how this apparatus and protocol permits multiple learning strategies in a minimally stressful environment, enabling finely graded analyses of cognition and emotionality. Male Sprague-Dawley rats living alone throughout adulthood adopted a sex-typic discernible spatial strategy. In contrast, males housed in group contexts utilized an algorithmic kinesthetic strategy, repeating the same motor action until they found food. Removal of food and distal, but not local cues, elicited anxious alertness, particularly in group-housed males. Cognitive performance of group-housed rats subsequent to food and cue removal was significantly impaired, yet enhanced in isolates.

Cache Decision Making: The Effects of Competition on Cache Decisions in Merriam's Kangaroo Rat (Dipodomys merriami)

Caching food is an economic, decision-making process that requires animals to take many factors into account, including the risk of pilferage. However, little is known about how food-storing animals determine the risk of pilferage. In this study, the authors examined the effect of a dominant competitor species on the caching and behavior of Merriam's kangaroo rat (Dipodomys merriami). The authors found that, as with conspecific competitors, kangaroo rats did not alter caching in response to the mere presence of a heterospecific competitor, but moved caches to an unpreferred area when the competitor's presence was paired with pilferage. These data suggest that Merriam's kangaroo rat assesses pilfer risk from actual pilferage by a competitor and adaptively alters cache strategy to minimize future risk.

The evolution of the cognitive map

The hippocampal formation of mammals and birds mediates spatial orientation behaviors consistent with a map-like representation, which allows the navigator to construct a new route across unfamiliar terrain. This cognitive map thus appears to underlie long-distance navigation. Its mediation by the hippocampal formation and its presence in birds and mammals suggests that at least one function of the ancestral medial pallium was spatial navigation. Recent studies of the goldfish and certain reptile species have shown that the medial pallium homologue in these species can also play an important role in spatial orientation. It is not yet clear, however, whether one type of cognitive map is found in these groups or indeed in all vertebrates. To answer this question, we need a more precise definition of the map. The recently proposed parallel map theory of hippocampal function provides a new perspective on this question, by unpacking the mammalian cognitive map into two dissociable mapping processes, mediated by different hippocampal subfields. If the cognitive map of non-mammals is constructed in a similar manner, the parallel map theory may facilitate the analysis of homologies, both in behavior and in the function of medial pallium subareas.

Unpacking the cognitive map: the parallel map theory of hippocampal function

In the parallel map theory, the hippocampus encodes space with 2 mapping systems. The bearing map is constructed primarily in the dentate gyrus from directional cues such as stimulus gradients. The sketch map is constructed within the hippocampus proper from positional cues. The integrated map emerges when data from the bearing and sketch maps are combined. Because the component maps work in parallel, the impairment of one can reveal residual learning by the other. Such parallel function may explain paradoxes of spatial learning, such as learning after partial hippocampal lesions, taxonomic and sex differences in spatial learning, and the function of hippocampal neurogenesis. By integrating evidence from physiology to phylogeny, the parallel map theory offers a unified explanation for hippocampal function.

Sex differences, but no seasonal variations in the hippocampus of food-caching squirrels: a stereological study

Recent studies have described sex differences in the relative size of the hippocampus that are associated with sex differences in space use in birds and short-lived mammals. A correlation between spatial learning and increased hippocampal volume has also been demonstrated in food-caching animals. Such results suggest that sexually dimorphic spatial learning (sex differences in space use during the breeding season) and seasonal variations in food-caching behavior (spatial memory for cache locations) might correlate with morphological changes in the hippocampus of adult long-lived mammals. We used modern stereological techniques to examine the volume and neuron number of the structures forming the hippocampal complex (dentate gyrus, CA3, and CA1) of wild adult eastern gray squirrels (Sciurus carolinensis) throughout the year. We observed differences in brain size between samples collected at different times of the year (October, January, and June). Our analysis showed sex differences, but no seasonal variations, in the volume of CA1 stratum oriens and stratum radiatum. There were no sex differences or seasonal variations in the relative volume or the number of neurons of any other layer of the structures forming the hippocampal complex. These results confirm the existence of sex differences in the structure of the hippocampus; however, this sexual dimorphism does not vary seasonally in adulthood and is likely to result from developmental processes. These results do not support the hypothesis that seasonal variations in food-caching behavior might correlate with morphological changes, such as variations in volume or neuron number, in the hippocampal complex of adult long-lived mammals.

The seasonal pattern of cell proliferation and neuron number in the dentate gyrus of wild adult eastern grey squirrels

The dentate gyrus is one of two areas in the mammalian brain that produces neurons in adulthood. Neurogenesis (proliferation, survival, and differentiation of new neurons) is regulated by experience, and increased neurogenesis appears to be correlated with improved spatial learning in mammals and birds. We tested the hypothesis that in long-lived mammals that scatter-hoard food, seasonal variations in spatial memory processing (i.e. increased processing during caching season in the autumn) might correlate with changes in neurogenesis and neuron number in the granule cell layer of the dentate gyrus (gcl DG). We investigated the rate of cell proliferation and the total number of neurons in the granule cell layer of wild adult eastern grey squirrels (Sciurus carolinensis) at three different times of the year (October, January and June). We found no seasonal differences in cell proliferation rate or in total neuron number in the granule cell layer. Our findings are in agreement with those of previous studies in laboratory mice and rats, and in free-ranging, food-caching, black-capped chickadees, as well as with current hypotheses regarding the relationship between neurogenesis and learning. Our results, however, are also in agreement with the hypothesis that neurogenesis in the dentate gyrus represents a maintenance system that may be regulated by environmental factors, and that changes in total neuron number previously reported in rodents represent developmental changes rather than adult plasticity. The patterns observed in mature wild rodents, such as free-ranging squirrels, may represent more accurately the extent of hippocampal plasticity in adult mammals.

The economy of winter: phenotypic plasticity in behavior and brain structure

Mobile animals must learn the spatial distributions of resources. The cost of foraging increases dramatically for temperate-zone animals during the winter. Two strategies may be used to balance the energetic budget: reducing costs of foraging and reducing need to forage. Both strategies are correlated with changes in brain structure, specifically in the hippocampus, a fore-brain structure used by birds and mammals to map spatial distributions of resources. Small mammals that reduce their need to forage, through hibernation or reduction in body size, show a specific reduction in the structure and size of the hippocampus. The costs of foraging can be also decreased by migration to better foraging conditions or by food-storing, both of which decrease the temporal heterogeneity of food resources. Both of these latter strategies are associated with increased hippocampal structure; for food-storing birds, this increase is a seasonal phenomenon. Thus not only behavior, but also learning ability and even brain structures in adult animals, may be phenotypically plastic in response to the changing demands of the environment.

Patient-controlled analgesia: a comparison of dosing regimens for acute postsurgical pain

This study compares several dosing regimens for patient-controlled analgesia (PCA) in the management of acute maxillofacial surgical pain. The dosing methods differed by presence or absence of an active drug (morphine [MS] vs saline), presence of a baseline infusion, and dose of drug delivered. Sixty-eight patients were enrolled in this prospective, randomized, double-blind, placebo-controlled trial that lasted 24 hours. The study was completed in two separate parts, each of which involved randomization of patients into four groups (part I) or three groups (part II). No significant differences were noted in pain scores in the preoperative, immediate postoperative, or 4-, 8-, 12-, or 24-hour periods among any of the groups, including the saline-only control groups; in either part I or part II of the study. Significant differences (P < .01), however, were noted in nausea and vomiting scores. Fifty percent (50%) of patients receiving MS vomited, while no patients in the saline groups vomited. This study calls into question the usefulness of PCA with MS in maxillofacial surgery patients. Pain control was questionable at best, and the rate of emesis was unacceptably high in patients with potentially compromised airways. Further research is required to determine if other analgesics provide better pain control with less nausea in the PCA system or if antiemetics can effectively be used to lower the incidence of nausea and vomiting.

Natural space-use patterns and hippocampal size in kangaroo rats

The size of the hippocampus, a forebrain structure that processes spatial information, correlates with the need to relocate food caches by passerine birds and with sex-specific patterns of space use in microtine rodents. The influences on hippocampal anatomy of sexual selection within species, and natural selection between species, have not yet been studied in concert, however. Here we report that natural space-use patterns predict hippocampal size within and between two species of kangaroo rats (Dipodomys). Differences in foraging behavior suggest that Merriam's kangaroo rats (D. merriami) require better spatial abilities than bannertail kangaroo rats (D. spectabilis). Sex-specific differences in mating strategy suggest that males of both species require more spatial ability than females. As predicted, hippocampal size (relative to brain size) is larger in Merriam's than in bannertail kangaroo rats, and males have larger hippocampi than females in both species. Males of a third species (D. ordii) also have smaller hippocampi than Merriam's kangaroo rat males, despite being similar to Merriam's in brain and body size. These results suggest that both natural and sexual selection affect the relative size and perhaps function of mammalian hippocampi. They also reassert that measures of functional subunits of the brain reveal more about brain evolution than measures of total brain size.

Sexually differentiated effects of radio transmitters on predation risk and behaviour in kangaroo rats Dipodomys merriami

In a 12-year study involving 191 radio-tracked Merriam's kangaroo rats and 337 subcutaneous radio implantations, females were killed by predators at a rate of 0.0054 per radio-bearing night and males at a rate of 0.0116. Both the mortality rate and the sex difference therein declined over the course of several nights after radio implantation. Females reduced their excursions from the day burrow for the first few nights after radio implantation, whereas males exhibited little if any such inhibition of movement. This sexually differentiated behavioural response to the transmitters is a likely source of the sexually differentiated mortality patterns.

Spatial memory and adaptive specialization of the hippocampus

The hippocampus plays an important role in spatial memory and spatial cognition in birds and mammals. Natural selection, sexual selection and artificial selection have resulted in an increase in the size of the hippocampus in a remarkably diverse group of animals that rely on spatial abilities to solve ecologically important problems. Food-storing birds remember the locations of large numbers of scattered caches. Polygynous male voles traverse large home ranges in search of mates. Kangaroo rats both cache food and exhibit a sex difference in home range size. In all of these species, an increase in the size of the hippocampus is associated with superior spatial ability. Artificial selection for homing ability has produced a comparable increase in the size of the hippocampus in homing pigeons, compared with other strains of domestic pigeon. Despite differences among these animals in their histories of selection and the genetic backgrounds on which selection has acted, there is a common relationship between relative hippocampal size and spatial ability.

Evolution of spatial cognition: sex-specific patterns of spatial behavior predict hippocampal size

In a study of two congeneric rodent species, sex differences in hippocampal size were predicted by sex-specific patterns of spatial cognition. Hippocampal size is known to correlate positively with maze performance in laboratory mouse strains and with selective pressure for spatial memory among passerine bird species. In polygamous vole species (Rodentia: Microtus), males range more widely than females in the field and perform better on laboratory measures of spatial ability; both of these differences are absent in monogamous vole species. Ten females and males were taken from natural populations of two vole species, the polygamous meadow vole, M. pennsylvanicus, and the monogamous pine vole, M. pinetorum. Only in the polygamous species do males have larger hippocampi relative to the entire brain than do females. Two-way analysis of variance shows that the ratio of hippocampal volume to brain volume is differently related to sex in these two species. To our knowledge, no previous studies of hippocampal size have linked both evolutionary and psychometric data to hippocampal dimensions. Our controlled comparison suggests that evolution can produce adaptive sex differences in behavior and its neural substrate.

Regional differences in normally occurring cell death in the developing hamster lateral geniculate nuclei

Normal cellular degeneration occurs in the lateral geniculate nuclei (LGN) of the hamster thalamus early in postnatal development. Degenerative debris can be observed in the ventral and dorsal nuclei at postnatal days 2-10 and is present in greater and more variable amounts in the ventral nucleus. Cell degeneration in the dorsal LGN is maximal at postnatal day 5, identical to the degeneration pattern of the hamster retina and superior colliculus, but shows a second peak at postnatal day 8 which may relate to the establishment of cortical connectivity. The incidence of degenerative debris is significantly higher in the peripheral margins of the dorsal nucleus, a pattern also seen in the retina and the superior colliculus, suggesting that a differential cell death may be involved in the formation of regional specializations in the visual system.